Emergence of structural anisotropy in optical glasses treated to support second harmonic generation

Few discoveries have puzzled the optics community more than the emergence of visible (green) light from optical fibers after strong irradiation by an infrared laser. This frequency-doubling phenomenon known as Second Harmonic Generation (SHG) is not expected to take place in a centrosymmetric material such as the amorphous silica fiber-core, which shows no measurable second-order optical susceptibility χ(2). The process to be efficient also requires well defined phase-matching between the interacting waves to allow for constructive interference, and this seems even more difficult to fulfill within the glassy medium. Several plausible explanations about the origin of the phenomenon have been put forward. One of the most widely accepted does not involve structural modifications to accomplish the breakdown of the glass radial symmetry. Rather, it postulates the emergence of a spatially modulated local dc field, E0, which, via a third-order nonlinearity (χ(3)) (finite in isotropic materials), induces a spatially modulated second order nonlinearity ( χ(2) is proportional to χ(3)E0 ) able to double the pump frequency. The achievement of a permanent χ(2) in optical glasses has focused a large research effort, which lead to the discovery of alternative poling techniques. In actual fact, the phenomenon can be produced by application of a high voltage (~5 kV, just below dielectric breakdown) to glass plates at moderate temperatures (~540 - 580 K, compared with ~ 1475 K where the glass melts). This method, known as "thermal" poling provides permanent second-order nonlinear responses comparable to those shown by inorganic crystals. Whether the mechanism(s) leading to the emergence of a second order non-linearity in "thermally-poled" glasses differ from those of photoinduced SHG or not needs to be clarified. At any rate, the relevant point stems from the possibility this method has opened up for developing inexpensive integrated optical frequency converters and electro-optic modulators